1. Field of the Invention
The present invention mainly relates to a method of measuring and adjusting exposure focus in a lithography process, and a method of manufacturing a semiconductor device using said method.
2. Description of the Related Art
Conventionally, an exposure apparatus (such as, a stepper and the like) is used for forming a fine element pattern of a semiconductor integrated circuit. The exposure apparatus performs projection exposure to form an image of a mask pattern on a reticle to a resist film having photosensitivity formed on a semiconductor substrate. The projected mask pattern is patternized as a pattern of the resist film by developing the exposed resist film. It is necessary to project the mask pattern on the reticle to a photoresist film without being out of focus on the resist film in order to obtain a resist pattern having designed predetermined cross sectional shape and dimension. As a technique to confirm a presence of focal shift state in a photolithography process of a semiconductor integrated circuit, a focal shift measurement method is described in Japan Patent Application Laid-Open No. 2005-012158.
Hereafter, outlines of the focal shift measurement method and a determination method for a best focus described in the prior document will be explained. In this technique, an isolated line pattern and an isolated space pattern are formed in a resist film, and a focal shift length is detected using a focal dependency of these pattern dimensions. FIG. 9 is a cross sectional view showing the isolated line pattern and isolated space pattern. In such technique, a top dimension 201 of an isolated line pattern 200, a bottom dimension 202 of the isolated line pattern 200, a top dimension 203 of an isolated space pattern 210 and a bottom dimension 204 of the isolated space pattern 210 are measured. The dimension to be measured in the isolated line pattern 200 is a line width, and the dimension to be measured in the isolated space pattern 210 is a space width.
FIGS. 10A and 10B are graphs showing focal dependency of the dimensions 201 to 204. FIG. 10A shows the focal dependency of the isolated line pattern 200, and FIG. 10B shows the focal dependency of the isolated space pattern 210. The horizontal axis of FIG. 10A indicates a focus value, and the vertical axis indicates a normalized edge inclination amount ΔLn of the isolated line pattern 200. The normalized edge inclination amount ΔLn is a difference between an edge inclination amount ΔL, which is a value obtained by subtracting the bottom dimension 202 from the top dimension 201 and an edge inclination amount ΔLo at the time of best focus, ΔLn=ΔL−ΔLo. Further, the horizontal axis of FIG. 10B indicates a focus value, and the vertical axis indicates a normalized edge inclination amount ΔSn of the isolated space pattern 210. The normalized edge inclination amount ΔSn is a difference between an edge inclination amount ΔS, which is a value obtained by subtracting the bottom dimension 204 from the top dimension 203 and an edge inclination amount ΔSO at the time of best focus, ΔSn=ΔS−ΔSo. In the graphs shown in FIGS. 10A and 10B, a focus value whose value in the vertical axis becomes zero can be regarded as best focus values for the patterns 200 and 210, respectively.
FIG. 11 is a graph showing focal dependency of tie sum of the normalized edge inclination amounts ΔLn and ΔSn (hereafter, referred to as a shift index) shown in FIGS. 10A and 10B. Herein, the horizontal axis of FIG. 11 indicates a focus value, and the vertical axis indicates the shift index. As shown in FIG. 11, the shift index normally indicates a change with a constant tilt with regard to the focus value. In the relationship shown in FIG. 11, a focus value whose shift index becomes zero can be regarded as a best focus value that satisfies both the isolated line pattern 200 and the isolated space pattern 210.
Therefore, in an exposure process for a semiconductor device, before exposing a pattern to a semiconductor substrate, at first, the top dimensions 201 and 203 and the bottom dimensions 202 and 204 of the isolated line pattern 200 and the isolated space pattern 210 are measured, respectively, and the results are calculated and the focal dependency of the shift index shown in FIG. 11 is acquired beforehand. Then, the top dimensions 201 and 203 and the bottom dimensions 202 and 204 of the isolated line pattern 200 and the isolated space pattern 210 formed on an occasion of the actual pattern exposure are measured and the shift index are calculated, and the result are compared with this graph. With this step, how much the actual focus value is shifted from the best focus value can be easily calculated. This method can easily and accurately detect a presence of a focal shift and a focal shift length and this is very useful as a focus measurement method.